1. Field of the Invention
The present invention relates to a composition for dental restoration which takes the advantages of both light-polymerization and chemical polymerization, and which is well cured by the irradiation of visible lights in a short period of time with no fear that it may be adversely affected by the intensity of light due to the transmission properties or transmission depth of light.
2. Statement of the Prior Art
Heretofore, the so-called chemical polymerization and light polymerization type restoration materials have been widely used in dentistry. The chemical polymerization type restoration materials are broken down into the powder. liquid type and the paste. paste type. Generally, the powder. liquid type restoration materials comprise a powdery component in which an organic peroxide such as benzoyl peroxide is added to a powdery organic such as polymethyl methacrylate and a powdery inorganic such as silica, and a liquid component in which N,N'-dimethyl-p-toluidine and the like is added to a polyfunctional monomer such as methyl methacrylate and/or dimethacrylate and/or trimethacrylate. In use, when the powder and liquid components are mixed together, the monomer is polymerized and cured by the oxidation-reduction reaction between the organic peroxide and the amine. On the other hand, the paste. paste type restoration materials are in the form of a paste composed mainly of fine powders and a binder resin for bonding together the powders. That paste is divided into two portions, one portion containing an organic peroxide and the other portion, an amine. In use, such pasty portions are mixed together, and the resulting mixture is filled in a cavity. Thereafter, the binder resin is polymerized and cured by the oxidation-reduction reaction between the organic peroxide and the amine within a certain period of time.
The disadvantage of the restoration materials of such types is that since the curing time is predetermined regardless of the will of an operator, it is required for the operator to rapidly finish his or her filling manipulation, when the predetermined curing time is short. In some cases, it is likely that the restoration materials may be cured before the filling manipulation is finished. It has also been pointed out that when the predetermined curing time is long, the physical properties of the restoration materials are adversely affected as a result of the fact that they are exposed to moisture or sputum in the oral cavity, while not cured. When it is intended to effect curing within a shorter period of time, it is required that the amount of the catalyst added such as an organic peroxide or amine be increased. Increases in the amount of the organic peroxide or amine added are practically unpreferred in dentistry, since coloration and discloloration tendencies become more marked and the amount of heat generated become more increased, correspondingly, and this is by no means preferred in practical dentistry. It has also have been pointed out that in the case of low-temperature polymerization or thin-layer polymerization, the polymerization reaction becomes incomplete, thus yielding an uncured mass having poor physical properties.
Turning to the light polymerization type restoration materials in which a photo-polymerization catalyst is used in lieu of the organic peroxide-amine base catalyst in the chemical polymerization type, their curing reaction does not proceed, unless they are exposed to light. This leads to the advantages that the manipulation and time therefor are optimized, because it is possible to substantially control the manipulation time and curing time according to the operator's will. Thus, the light polymerization type restoration materials have recently found increased use in dentistry.
A number of the compositions for the light polymerization type of dental restoration materials are well-known in the art. In view of their safe use in the oral cavity, however, use is often made of the visible light polymerization type restoration materials in particular. Compositions relating to the method using the light curing catalysts disclosed in British Patent No. 1408265 specification, i.e., the .alpha.-diketone base camphor quinone and amine reducing agents are used in virtually all products. The visible light curing type restoration materials are cured upon exposure to light falling under the wavelength range of 400 to 500 nm, but do not provide any satisfactory cured product in a wavelength region of below 400 nm.
The disadvantages of the light polymerization type restoration materials are that their curing reaction is easily affected by the transmission properties and intensity of light, and becomes insufficient, as the portion to be irradiated is far away from the light source used, or the transmission properties drop, leaving a large amount of residual unreacted monomer. There are thus drops in the physical properties such as hardness, strength and the like, which are unpreferred from the clinical point-of-view. When the material is opacified by the addition of an opaque component and the like, it is cured only on the outer layer, and is not possible to be set deep inside. These light-polymerizable compositions for dental use are typically disclosed in our co-pending application Ser. No. 906196, 906283 and 906285.